Electric psychometer



Dec. 26, 1.950

P. L. WILHELM ETAL 2,535,249

ELECTRIC PSYCHOMETER Filed Feb. 26, 1948 2 Sheets-Sheet 1 Dec. 26, 1950 P. L. WILHELM ETAI. 2,535,249

ELECTRIC PsYcHoMETER -Filed Feb. 26, 1948 2 Sheets-Sheet 2 Patented Dec. 26, 1950 ELECTRIC PSYCHOMETER Paul L. Wilhelm, Michigan Township, and Frank Donald Burns, Michigan City, Ind.

Application February 26, 1948, Serial No. 11,264

9 Claims.

j This invention relates to improvements in electric psychometers, sometimes known as lie detectors. The present device utilizes the phenomenon that the resistance of the skin of an individual changes with an emotional change in the human body. We are aware that devices for detecting dishonest from truthful statements of an individual have been provided. These devices, to the best of our knowledge, usually have responded to cardiac and pneumatory reaction of an individual. Such prior devices, while effective for their intended purpose. have been subject to various disadvantages and limitations. Among' such disadvantages have been the high cost thereof, the delicate construction thereof, and the'large size and heavy weight thereof which has tended to preclude desired portability. Other disadvantages of previous devices have been that they have not been adjustable as to sensitivity according to the personal reactions of different individuals in most cases, and, further, that the attachments to the body of the individual necessary for their use have produced discomfort of the subject and thus required that the tests of the subject by use of the device be limited as to time.

VThe primary objects of this invention are to provide a device of this character which is comparatively inexpensive so that it may be available for use 'by law `enforcement and other agencies having limited funds, which is light in weight and portable so that it may readily be transported from place to place, and which is rugged in construction to resist damage incident to use thereof and to transportation thereof.

A further object is to provide a device of this character with means for adjusting the sensitivity of response thereof so that the variations in the reactions of different individuals may be compensated and accurate readings secured for all subjects, and which is further provided with convenient means for calibrating the device.

A further object is to provide a device of this character which may use a sensitive but comparatively rugged measuring element, such as a milliammeter. and which includes means to protect the measuring deviceautomatically in the event that the current being measured exceeds the range of the instrument during use.

A further object is to provide a device of this character with means adapted to be worn by thevsubject for detecting variations in skin resistance, which are of novel construction. which may be applied and removed -easily and quickly, and which may be wornfor extended periods ol (Cl. 12S- 2.1)

time to facilitate extended questioning of the subject without discomfort to the subject. y p

A further object is to provide a device of this character which can be used by individuals havn ing little familiarity with electrical apparatus without requiring extensive personal instructions.

to such individual, and which is constructed from standard electrical parts readily available` at firms dealing in electrical and radio equipment.

A further object is to provide a device ofthis.

character which will not be damaged by accidental energization thereof over long periods of time and in which no danger exists that the subject will be harmed or injured by electrical shock.

Other objects will be apparent from the vfollowing specication.

In the drawing: Fig. 1 is a front elevation of the instrument mounted in a case and provided with indicating* Fig. 5 is an end view ofthe skin contacting"l electrode used in the device. I

Fig. 6 isa longitudinal sectional view taken on line 6 6 of Fig. 5. f

Fig. 7 is a circuit illustrating the mannerv in.

which the various elements of applicants device are connected, to provide a modified Wheatstone bridge.

Referring to the drawing, and particula-rlytov Fig. 2 thereof which illustrates one embodiment of the invention, the numeral Ill designates the terminals to which the Skin engaging device, such as the device shown in Figs. 5 and 6, is adapted to be connected. One of these terminals l0 is connected by a lead l2 with a resistor Ill, and the resistor` I4 is connected in series with a resistor;

I6 by means of a lead i8. The resistor I6 in turn is connected with a resistor 20 by means of a lead 22. TheY resistor 2U is connected by a lead 24 in series with a battery 26. A second battery 28 is connected in series With-the battery 26 by a lead:

30. and a lead 32 vbranches from the lead 3U between the two batteries 26 and 28 and is tapped. to the conductor or lead I8 between the resistors I4 and i6. A resistor 34 is interposed in a lead. 3B tapped tothe lead I2 at 38. The lead 3 6'will include a flexible portion which terminates in a slider member adapted to engage the resistance 20 and cooperating therewith to provide a variable resistor for zero adjustment or centering. The second terminal Ill is tapped to the lead 3,6 between the resistance 34 and the terminal 40 by a lead 42.

A slider 44 for sensitivity adjustment cooperates with the: resistance '24 and is connectedby a lead 46 with the grid 48 of a signal 'amplifying electron emission tube 50. The cathode 52 of the tube is connected by a lead 54 with a second zero adjusting or centering slider element 56 cooperating with the resistor I6. In the preferred form the circuit to the cathode will include a bat- I tery 58. l The plate 60 of the tube Bill is connected by v la lead 62 with a plate battery 64, there preferably being a xed resistor 66 connected in the lead 62 between the tube and the battery 64. The lead tt connects the battery Gi in series with thebattery 2li. A lead Hl is tapped from the lead 62 at the terminen of the pla-te tu and between the plate l and the resistor 65. This lead 'I0 servesto connect a measuring instrument, Stich as a milliainmeter 12 or a recorder, in the clieuitin parallel to the resistor G6 and the battery 64 with its opposite end being tapped at Il to the lead 6B. rThe coil 1B of a relay is Ainterposed in the lead 'l0 and the armature 'lil of the relay constitutes or` controls a switch interlri a 'shunt circuit 80 in 'parallel with the measuring eiement 12.

The elements which respond to the skin resistance of the subject are best illustrated in Figs. e

5 mld 6 l'l. comprise tw() puits; Olie bart is preferably in the form of a ring or short tube 82 formed of conductive material, the `diameter of whose bore is slightly greater than the inner peripheral diameter of a nger ring of the largest size offered by jewelers; The other part cornprises en elongated conductive metallic plate 84 ol' concave-convex form in transverse cong'uration and provided at the center of its convex falce with a longitudinal integral rib 86 of tapered flil-L A exble electrical lead 9D is electrically connected with a plug 92 whichAiitswithinp a socket formed in the electrode 84, B6'. -In the preferred form the socket will be constructed i illustrated in Fig. 6 with a large diameter portion B4 thereof adapted to receive the large portion of a 'plug 92 and with a small diameter inner socket portion 96 slightly eccentric with to the socket portion B4 and adapted to receive a reduced terminal projection 'of the The device is adapted to be mounted in a housing or casing IBG-, as best illustrated in Fig. 1. This housing will mount sockets |02 adapted to detaohabiy receive plugs (not shown) which are mounted upon the lead 80 to the electrodes. The sockets I 02 will constitute the terminals I0 illustrated in the circuit inKFig. 2. The casing |06 will mount the measuring member 'or millimmeter 12 in its iront Wall to be readily visible by the operator. A plurality of knobs 104, |06 and |08 will be mounted upon the `casing |09.. These knobs will be connected respectively with the slider 44, 'the slider 56 and the `slider 40; to control the adjustment of said sliders. The front panel mounting of these knobs permits their 'use while viewing the meter 12 to facilitate the adjustment 'and control of the operating cir cuit.

iinger electrodes are employed, 'being mounted upon different fingers and each of which has only one lead connected thereto, the circuit being completed through the skin of the subject, It will be apparent that the finger electrodes can be used very simply by sliding the retainer ring 82 over `the finger of a subject and then sliding the elongated electrode 84 within that retainer ring to a position in which it engages they skin of the 'userwith a positive but comfortable electrical contact.

vIt will be observed that the circuit is essentially a Wheatstone bridge circuit in which the resistors I4, I6 and 20 constitute three of the legs of the circuit and the electrical resistance of the skin of thev subject forms the fourth resistor in the circuit. The resistor 34 is connected in parallel. With the resistance formed by the skin of the subject. The output of the bridge is conducted through the leads 4B and 54 to the electronic tube 5U by which the current is amplied to a value sufficient to energize the measuring device 12. The current values in this instance can be kept at a very low level, thus insuring against electrical shock or injury to the subject and yet are sufficiently high to permit the use of. a fairly rugged type of electrical instrument, such as a mlllammeter, as the measuring means. In this connection, the amplification. by the electron tube is suiiicient to raise the signal strength for actuating the meter 'l2 `from a very small value to which only a galvanometer would be responsive to the higher value, thus making it possible to use the comparatively rugged 4milliaxnmeter` in place of a simple galvanometer;

One of the outstanding advantages of the device is that it may be adjusted to compensate for differences in the skin resistance of diierent subjects by varying the position of the slider 44 upon the resistance 34. Thus in instances Where the skin resistance ofI twosubjeets vary, the slider can be adjusted during initial preliminary testing with each subject to establish the conditions required for use as to that subject, thus setting up the instrument to give, upon the metering element 12, indications which are equal in u amplitude to those of a subject whose skin resistance is at a substantially different electrical value. A second important feature of this device is the provision of means for centering or balancing the instrumenta Thus, whenever any adjustment of the sensitivity slider 44 is made, lit will affect the balance of the bridge and the zero positioning of the pointer of the milliammeter 12. The sliders 40 and 56 constitute these centering means. One thereof may be provided for relatively coarse adjustment, and the second thereof may be constructed for relatively vfine adjus'ti'rler'it.A Ifh'is by the manipulation oi' the proper ones oi the knobs NJB and lcontrolling the sliders 55 and 4i), the indicator needle can quickly be restored to a zero position.

Y Another important advantage of the device is the provision of the relay 16,118 for controlling the shunt circuit ill). relay can be so constructed that in the event the current flow through the une 1u 'to 'the meter 1i reaches or exceeds the maximum current whichthe nstru`' ment will carry, the relay will operate "to close the normally open shunt circuit 8U, thereby preventing the possibility that the meter 12 will be subjected to any damaging current i'iow condiibh.

The, ability ofthe device to be operated by batteries, the simplicity 'or the skin contacting electrodes, the simplicity of the Aadjiisinxuaiits of the operating circuit, all make for conipactr lIn the use of the device two electrode as illustrated in Figs. 5 and 6, are fastenedon two lingers of the subjectfbeing connectedqin= the circuit as the sockets |02. A small amount of electric current is passed through thelby dy while the resistance of the body is balanced by the "adjustable resistor 4 4. in the bridge lcir it. In other words, after the individual has placed at rest after the Aelectrodes l.have

justed by manipulation, ofl the'proper one f the the other knobs which servev controljthe rheostats 40 and 56 toacciirately center the 1fil.

sition of the needle-of the Vmeter 12. The latter adjustments are4 preferably performed'f ljby moving said sliders to a zero position and vthen progressively shifting. them luntil the desired reading is indicated upon the meter l2, that" a centered reading. The sensitivity settingjcan then be checked by having the subject take va deep breath which will produce a certainfa'ctiiation of the meter under normal conditions. Ii the meter operation exceeds or falls below"`this normal, a change in the setting of the slider44 may be made, followed by changes in the setting of the sliders 40 and 56 for centering purposes, and the sensitivity can then be checked again. v

In testing a subject it is desirable that aiixed set of questions which can be 4answered Yes or No should be prepared in advance, certain of which relate to nonpertinent matters, and notes can be made of the readings of the meter 1,2 in conjunction with vthe answering of such nonpertinent questions. Such reading can' be utilized to establish the normal reaction of the subject to questioning. Several of the questions interspersed,v among the list to be asked will` `per' tain to the subject mattei' of' the investigation. L t is desirable that this list of questions be asked of .the subject more than once but preferably not more than three times, and notes should be taken of the'readings applying to each question on each occurrence of its asking.' It is important that 'suincient time be allowed betweenanswers to permit the needle or indicator ofltlie measuring unit l2 to return to a rest position normal to the subject before another question is asked. During the questioning the subject should remain quiet. and relaxed, and his atten'- tionl should not be diverted from the questions by any. external means, and preferably not by any sound or noise. The results of the readings must be interpreted from experience and any possibilities4 that an abnormal reaction has been'v caused by any condition, except an untruthful answer, must be ascertained. Thus if physical' movement has occurred, or if interference has occurred, or if it appears that the subjectis frightened by the machine, orv if it appears that the question asked may be creating a mental association in the mind of the subject with 4a fact situation different from that being investigated, suchconditions may account for abnormalityof the responseor reaction of the subject. In the interpretation of thereadings itmust alsobe borne in mind thatthere is no setof abnormalv reactions to-indicate that a lie has been told, and the operatprthrough experience rcan readily judge when a lie has been told, according to the difference between a normal reaction and an abnormal reaction caused by lying. As a general rule, however, it will be found that, where the subject lies, the abnormal reaction shown will be at least twice as much as the normal reaction on the same test. In other words, if normal reaction of the subject gives meter readings from zero to three, then meter readings of six or more tend to indicate that a lie has been told.

`The circuitshown in Fig. 3 differs from the circuit shown in Fig. 2 essentially by the fact that two stages of ampliiication are provided instead of the single stage of amplification as shown vin Fig. 2. The parts of Fig. 3 which are similar to the `parts of Fig. 2 bear the same reference numerals. The second stage ofzamplication is provided by an electron tube ||0 whose grid ||2 is connected by a lead |4 tapped` from the lead 62 at a point between the plate 60 of thevfirst tube 50 and the resistor `66 in lead 62. The cathode ||6 of the tube Ill) is connected by a lead |I8 with lead 3i) between the battery 26 and a battery |20. The lead 32 in this circuit is tapped with the lead 30 ata point between the battery |20 and the battery 26. The cathode circuit includes the battery |22 similarV to the .battery 58 used with the electron tube 66. The plate |24 of the tube ||0 is connected by a lead |26 with the lead 62 between the resistance 66 and the battery 64 in the lead 62. The lead |25 has a resistance |28 interposed therein. The lead l0 is connected-with the lead |26 between the resistance |28 and the tube plate |24 and extends in parallel with the battery 64 having its opposite end connected with the lead 68 between the batteries 64 and 28. The relay 76, 78 is provided in conjunction with the lead 'l0 and the shunt lead 80 in the same manner as in the Fig. 2 embodiment, said shunt lead 80 be adapted for connection across the opposite terminals of the measuring meter l2. This embodiment of the invention possesses all of the advantages and is constructed to operate in the same manner as the device shownin Fig. 2, difber of stages of amplication.

Fig. 4 illustrates a device which is adapted to be actuated from an alternating current source with which the leads |30 are connected. The power circuit |30 has the primary coil |32 of a transformer interposed therein.

fering therefrom solely with respect to the num-Y in in series relation. A lead |52 is tapped from* the lead |46 between the cathode 40 andthe resistance |48 and has a filtering condenser |54 interposed therein. The lead |52 is connected with the lead 24 of the psychometer circuit. 'Iv'hey second terminal of the secondary coil |44 is 'con-f nected by a lead |56 with the lead |52 to which it is tapped at |58 between the cathode |40 and the' filtering condenser |54. A lead |60 is tapped to the lead |46 between the resistances |48 and'l50A and is tapped at its opposite end to the lead |52 between the condenser |54 and the point of con 'I nection oflthe lead |52 with the'psychometer'` I The transformer may be of any suitable type .and prefer' of. mamas-in; @revised ey umm 01mm. 'me leed ITW iise nite'ng desf mi: ifrosedf. tiefem.

m ms. embodiment' sf wiiei psyehmseri am; s Substanz-many' the 'same ae mit; shewfi 3 reversing: the efr iw@- the 15u-be 50 the@ @He tien has n iustrafted arid" dessi-'ieee tiefem, it will" b derstood that ciinge's m'y be z'iide said circuit sind' inciudinginut and outum'eads' bridge electrode means adapted for ngsgeriieiili D ifig sein. meten 4. An electric psychometer comprising an el 9 Y connected at one input lead and in series with the resistance element of the third resistor and with the slider of that one of the rst named resistors connected to the other input lead, one output lead being connected to the slider of the other of said first named resistors, the other olitput lead being connected to the slider of the third resistor.

9. In an electric psychometer, a Wheatstone bridge circuit including a pairof input leads and a pair of output leads, a pair of variable resistors having resistance elements connected in series between said input leads and each having a slider, a third variable resistor having a slider and a resistance element, a fixed resistor connected at one input lead and in series with the resistance element of the third resistor and with the sliderof that one of the rst named resistors connected to the other input lead, one output lead being connected to the slider ofthe i 20 other of said rst named resistors, the other output lead being connected to the slider of the third resistor, and electrode means connected to said bridge circuit in parallel to the resistance element of the third resistor, said electrode means being adapted for engagement with the. skin of a person whereby the resistance of the skin influences said bridge circuit.

PAUL L. WILHELM.

FRANK DONALD BURNS.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 1,761,476 Hathaway June 3, 1930 2,110,392 Dorr Mar. 8, 1930 2,398,933 Raesler Jan. 19, 1943 2,339,579 Milne et al Jan. 18, 1944 2,379,955 Eilenberger e July 10, 1945 

